1. Field of the Invention
The present invention relates to a drive controller of a lens apparatus used, for example, for picking up a picture for television, in particular, of a lens apparatus capable of performing manual focus.
2. Related Background Art
An optical system of a business-use lens apparatus (hereinafter, referred to as a TV lens), for example, used for picking up a picture for television, generally in the order from an object to an image, includes a front lens unit for performing focus adjustment, a power-varying lens unit including a variator for performing power-varying and a compensator for compensating movement of an image plane due to the power-varying, and an image-forming lens unit for forming an image. Upon bringing an object into focus, manual focus is performed for driving the front lens unit by mechanically transmitting a manual operation or electrically driving it in response to the manual operation.
This is because a professional camera operator operating a TV lens not only merely focuses the lens on a subject but also often intentionally defocuses it with an intention of achieving an imaging effect, whereby the operator wants to have a choice of manual focus.
As such, many lenses having a higher resolving power and a smaller depth of field than ever before thanks to prevalence of a high definition (HD) camera are used.
When a TV lens having a small depth of field is used, a picture-pickup operator has difficulty of determining whether focusing adjustment is achieved with the help of an image appearing in a small viewfinder provided in the camera. Accordingly, even when the pickup operator believes that the focusing is achieved by manual focus, the operator may find slight focal shift upon checking a recorded image.
In order to solve such a problem, a TV lens having an auto-focus function similar to a consumer-oriented one, in addition to having a manual focus function, so as to adjust a focal position of the manual focus with a variety of functions is recently proposed.
For example, Japanese Patent No. 3412713 proposes a focusing method in which focusing adjustment is performed such that a focal-point evaluation value obtained from a picture signal during manual focus is stored, and, after completion of the manual focus, the lens unit moved by the manual focus is driven to a position at which the stored focal-point evaluation value is maximum (see Columns 0012 to 0015, FIG. 1, and so forth).
Unfortunately, with the method disclosed in the above patent document, since the lens unit driven to a position at which the focal-point evaluation value is maximum is identical to that driven during the manual focus, a clutch mechanism for selecting one of drive systems for the manual focus and for afterward focus adjustment is needed, thereby leading to a complicated structure of the lens apparatus.
A drive mechanism different from that including the clutch mechanism, having a structure in which a focus operation member is endlessly rotatable, for example, a drive mechanism of an electronic ring type, generally used in a consumer-oriented video camera and having a structure in which an operation quantity of the focus operation member is detected by an encoder or the like, and a focus lens is driven by a drive member such as a motor in accordance with a detected quantity can make the structure of the lens apparatus simple.
Unfortunately, since a professional camera operator often picks up a picture while making guesswork of a focal position from an extremely close end to an infinite end, relative to the operation end of the operating member, endless rotation of the operating member causes a problem that picking up a picture is difficult.
In a consumer-oriented video camera, as an auto-focus system, a rear focus system in which a lens unit closer to an image than a power-varying lens unit is generally used. Whereas, in a TV lens different from the consumer-oriented one, since its variable power ratio and the image size of an image pickup element are great, a large space is needed for achieving the entire focus by using only a rear focus lens unit. In the meantime, the TV lens has a structure in which an extender or the like can be inserted so as to shift a focal position of the overall lens system, whereby keeping a space allowing the rear focus lens to move is difficult.